The present invention relates to a cross-linkable or cross-linked rubber composition having improved hysteresis properties in the cross-linked state, which is usable to constitute a tire tread or to be used therein. The present invention further relates to a process for the preparation of such a cross-linkable rubber composition, to a tread comprising the cross-linked rubber composition, and to a tire having reduced rolling resistance.
Since fuel economies and the need to preserve the environment have become priorities, it has become desirable to produce mixes having good mechanical properties and as low a hysteresis as possible so that they can be processed in the form of rubber compositions useful for the manufacture of various semi-finished products involved in the make-up of tires, including, for example, underlayers, sidewalls or treads. It is also desirable to obtain tires having reduced rolling resistance.
To achieve such objectives, numerous solutions have been proposed, consisting of, in particular, the modification of the structure of diene polymers and copolymers at the end of polymerization by means of functionalizing, coupling or starring agents. The great majority of these solutions have focused on the use of functionalized polymers, which are active with respect to carbon black, with the goal of obtaining a good interaction between the modified polymer and the carbon black.
By way of illustrating the prior art relating to reinforcing fillers formed of carbon black, mention may be made of U.S. Pat. No. 3,135,716, which describes the reaction of living diene polymers at the end of a chain with a polyfunctional organic coupling agent in order to obtain polymers having improved properties. Mention may also be made of U.S. Pat. No. 3,244,664, which discloses the use of tetra-alkoxysilanes as a coupling agent or starring agent for diene polymers.
Silica has been used as a reinforcing filler in cross-linkable rubber compositions, in particular those compositions intended for use in tire treads, for a long time. However, this use has remained very limited because of the unsatisfactory level of certain physical properties of such compositions, in particular, abrasion resistance.
Thus, it has been proposed, in order to overcome these drawbacks, to use functionalized diene polymers, instead of the non-functionalized polymers which were used before, and in particular polymers functionalized by alkoxysilane derivatives, such as tetraethoxysilanes. For example, mention may be made of U.S. Pat. No. 5,066,721, which describes a rubber composition comprising a diene polymer functionalized by an alkoxysilane having at least one non-hydrolyzable alkoxyl radical, which makes it possible to eliminate the polymerization solvent by steam stripping.
One disadvantage of these functionalization reactions lies in the coupling reactions which accompany them, which generally make it necessary to use an excess of alkoxysilane and/or to employ intensive mixing, in order to minimize these coupling reactions.
Another drawback of these reactions lies in the later implementation of the steam stripping operation, which is necessary to eliminate the polymerization solvent. In fact, generally, experience shows that the functionalized polymers obtained undergo changes in macrostructure during this stripping operation, which result in serious degradation of their properties, unless one is limited to using as the functionalizing agent an alkoxysilane belonging to a restricted family, such as that described in the aforementioned U.S. Pat. No. 5,066,721.
Consequently, it may be seen from the above that the use of diene polymers comprising an alkoxysilane function to obtain rubber compositions comprising silica as reinforcing filler is not satisfactory, despite the improved physical properties of these compositions.
Thus, research has been conducted on other functionalization reactions, always with a view to obtaining such rubber compositions.
By way of example, mention may be made of French Patent No. FR 2.740.778, assigned to the Assignee of the present invention, which discloses the incorporation, into rubber compositions that comprise, as the reinforcing filler, silica in a majority proportion (for example, a blend of silica and carbon black), of diene polymers bearing at the chain end a silanol function or a polysiloxane block having a silanol end. For example, a functionalizing agent consisting of a cyclic polysiloxane, such as hexamethylcyclotrisiloxane, may be used. The functionalized polymers obtained can be separated from the reaction medium, resulting in their formation by steam extraction of the solvent, without their macrostructure and consequently their physical properties changing.
Mention may also be made of European Patent No. EP 0 877 047, which discloses the incorporation of such polymers having a silanol function in rubber compositions comprising as the reinforcing filler carbon black having silica fixed to its surface.
It has been possible to establish that these polymers impart rubber properties, in particular, hysteresis and reinforcement properties, in the cross-linked state, which are improved compared with those of control compositions based on non-functionalized diene polymers, and which are at least analogous to those of compositions based on diene polymers comprising an alkoxysilane function.
Mention may also be made of European Patent No. EP 0 692 493, which establishes that diene polymers bearing at the chain end alkoxysilane groups and an epoxy group result in improved reinforcement properties and in reduced hysteresis losses at small and large deformations.
One disadvantage of these polymers, which comprise a functional group that is active for coupling to silica or to carbon black surface-modified by silica, is that the improvement in the hysteresis and reinforcement properties which they impart to the rubber compositions incorporating them is generally accompanied by a processing ability of the non-cross-linked mixes which is compromised when compared to the processing ability of non-functionalized xe2x80x9ccontrolxe2x80x9d polymers.
Among the other functionalization reactions studied, mention may be made, for example, of the functionalization of the diene polymers along the chain by COOH functions.
The functionalization along the chain can be effected by direct metallation, in the presence of N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine (TMED), by means of butyllithium or metallic sodium (as described in U.S. Pat. Nos. 3,978,161 and 3,976,628, respectively), followed by a carbonation reaction by means of carbonic gas. However, such a process has the disadvantage of generally resulting in cuts in the chain of the modified polymer.
Two specific reagents, of the respective formulae HSCH2CO2CH3 and N2CHCO2CH2CH3, have also been used to graft COOH functions along the chain of a diene polymer. For the description of the reaction mechanisms relating to the use of these two reagents, reference may be made respectively to the following documents: Sanui et al., J. Poly. Sci., Polym. Chem. Ed. 12:1965 (1974) and Tanaka et al., J. Poly. Sci., Polym. Chem. Ed. 17:2975 (1979). However, one major disadvantage of using one or the other of these two reagents is that it results in significant changes in macrostructure for the modified polymer.
This functionalization along the chain may also be implemented by means of carbon monoxide, either by hydroformylation followed by oxidation of the aldehyde formed (as described in U.S. Pat. No. 4,912,145), or by direct hydrocarboxylation of the polymer (as described in Ajjou et al., Macromolecules 29:1784 (1996)). The catalysts used for these reactions are based on rhodium or palladium. One disadvantage of this functionalization by carbon monoxide lies, on one hand, in the drastic nature of the operating conditions and, on the other hand, in the frequent formation of a gel in the reaction medium.
Functionalization by means of maleic anhydride is more widespread. This type of functionalization makes it possible to obtain succinic anhydride units along the chain, which are precursors of the COOH functions. Reference may be made to U.S. Pat. Nos. 4,082,817 and 4,082,493 for examples of implementation of such functionalization. However, this method of functionalization may also result in the formation of a gel.
The use of diene elastomers comprising COOH functions along the chain for the preparation of rubber compositions useful in tires, is disclosed, in particular, by U.S. Pat. No. 5,494,091. This document discloses a rubber composition filled with carbon black comprising 25 to 55 phr (where phr represents parts by weight per hundred parts of elastomeric matrix) of polyisoprene and 45 to 75 phr of a diene polymer belonging to the group consisting of homopolymers of conjugated dienes and copolymers of conjugated dienes with mono-olefins, such as EPDM terpolymers (of ethylene, propylene and a diene), where part of at least one of these polymers comprises COOH functions grafted along the chain by reaction with a metal salt of unsaturated carboxylic acid, for example zinc dimethacrylate. The composition thus obtained is supposed to have a sufficiently high rigidity to be used in an internal reinforcement rubber for tire sidewalls, so as to permit travel with a flat tire under satisfactory conditions.
The present invention relates to the unexpected discovery of a cross-linkable or cross-linked rubber composition having reduced hysteresis losses at small and large deformations, which are similar to those hysteresis losses of known compositions based on polymers comprising functional groups which are active for coupling to silica (such as the alkoxysilane or silanol groups mentioned above). The cross-linkable or cross-linked rubber composition of the present invention also has processing properties in the non-cross-linked state which are improved compared with the processing properties of known compositions filled with silica. The processing properties of the rubber composition of the present invention are also comparable to the processing properties of compositions based on non-functionalized polymers that are filled with silica.
The cross-linkable or cross-linked rubber composition of the present invention is obtained by the association of at least one diene elastomer having a molar ratio of units originating from conjugated dienes greater than 30% and comprising carboxylic acid functions along its chain with a reinforcing inorganic filler.
The advantageous characteristics of the rubber composition of the present invention make such a composition useful to constitute a tread for a tire. Thus, the present invention further relates to a tire and a tread for a tire comprising or made of the rubber composition of the present invention. The present invention also relates to a process for forming the rubber composition of the present invention, described in more detail below.